How many steps do you walk each day?

If you live in the U.S. and can claim more than 4,774 steps daily, you're exceeding the average American's total. Comparable figures from England and Japan are 5,444 and 6,010 daily steps, respectively — with this info derived from cellphone data.

The distance that individuals walk daily varies, with a host of factors affecting the total, including where you live, what work you do and whether you are able to walk at all. In a riveting book published Tuesday, Exercised: Why Something We Never Evolved to Do is Healthy and Rewarding, Harvard University evolutionary biologist Daniel E. Lieberman cites the step data to make a point. Many of us today are fairly inactive (under 5,000 steps is defined as "sedentary"), so much so that our activity levels are mismatched with the way we evolved.

Exercise can be understood as "discretionary, planned physical activity for the sake of physical improvement." In other words, when we work out, we set aside time to exercise and do it because we have goals related to better bodies and better health.

Our ancestors never did any such thing. Certainly, they moved a lot in the course of their daily lives. We know this because the hunter-gatherer lifestyle — making a living off the land, without farming — demands it, and also because data from contemporary hunter-gatherers like the Hadza Indigenous people of Tanzania give us some clues to past activity patterns. Hadza individuals are about 12 times more active than the average American or European. Elderly people remain highly active, as they must have in the past. "Almost no one in the Stone Age," Lieberman notes, "least of all grandparents, managed to avoid hours of walking, running, digging, climbing, and other manual labors."

Yet, here's the central point: There was no exercise in the Stone Age! People had high activity levels simply because that's how it was possible to survive by hunting and gathering.

With that distinction in place, Lieberman begins a process of myth-busting about exercise. Some exercise enthusiasts, he writes, insist that "we were bornto exercise because for millions of years our hunter-gatherer ancestors survived through walking, running, climbing, and other physical activities." On the contrary — and here comes a surprising but scientifically sensible statement — we evolved to limit our physical activity whenever we reasonably could. With all the survival tasks requiring our energy, the last thing needed ancestrally was to set aside time to work out.

An irresistible aspect of Exercisedis Lieberman's firm stance that no shame or stigma be attached to those who find it challenging to sustain an exercise program: "So if, as you read these words, you are seated in a chair or lounging in bed and feeling guilty about your indolence, take solace in knowing that your current state of physical inactivity is an ancient, fundamental strategy to allocate scarce energy sensibly."

Indolence is not goodfor us, of course. Sitting for long hours, for instance, encourages inflammation throughout the body and is associated with chronic disease. This is especially true of the way we tend to sit in our culture — on a chair — compared to cultural traditions of squatting, kneeling or sitting on the ground, which causes the muscles to be more highly active. Yet if you're attached to your chair or couch, all is not lost: In one study, in which participants interrupted their sitting with just 100 seconds of movement every half-hour, the result was lower blood levels of sugar, fat and bad cholesterol.

Another exceptionally informative part of the book discusses the damage-and-repair cycle brought on by exercise. Lieberman explains more clearly than I've ever read what exercise does to the body and how the body then begins to repair itself afterwards. Briefly put, exercise unleashes a flood of waste products and unstable oxygen molecules that compromise cells' function. Following a workout, the body slowly begins to repair the damage, through a suite of anti-inflammatory and cleanup responses. Often, this repair process works so well that it leads to a state of better health in the body overall.

This beneficial process evolved in an evolutionary context of physical activity, as carried out by our ancestors. Thus, repair today does not happen efficiently if you are sedentary: "We never evolved to activate these maintenance and repair responses as effectively in the absence of regularly physical activity." Additionally, an increase in BDNF (brain-derived neurotrophic factor) that helps protect our brains against Alzheimer's disease increases with physical effort, for the same evolutionary reason. Insights like these alone make reading the book worthwhile.

To integrate exercise into our lives in sustained fun ways is a goal we should all have, Lieberman says, though he appropriately explains that certain disabilities may make this difficult or impossible. We can bring this about by exercising socially, for example; our species is gregarious by nature and many people enjoy collective activity, plus, if we commit to an exercise meetup, we're less likely to succumb to that urge not to exercise. (The book was mostly written before the COVID-19 pandemic forced temporary suspension of shared exercise activities in the name of good public health.)

The oft-cited recommendation to exercise 150 minutes a week Lieberman doesn't discount, but he emphasizes there's no optimal routine of exercise type or quantity because we all have different bodies and circumstances. A good across-the-board summary is this: "Make exercise necessary and fun. Do mostly cardio, but also some weights. Some is better than none. Keep it up as you age."

Exercised is written in a warm, sometimes dryly amusing tone that's highly appealing. Colorful personal stories enliven the book. Follow along as Lieberman accompanies two Hadza men on a hunt, rides a dog sled across icy terrain Greenland and tries to outrun a horse in a race in Arizona.

Lieberman makes a superb guide for anyone wishing to understand why it can be hard to commit to exercising and why we should do it anyway.

Barbara J. King is a biological anthropologist emerita at William & Mary. Her seventh book,Animals' Best Friends: Putting Compassion to Work for Animals in Captivity and in the Wild, will be published in March. Find her on Twitter@bjkingape.

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